Two genes involved in the determination of segment identity in Drosophila were studied: female (1) sterile homeotic (fsh), and trithorax (trx). The fsh gene is required maternally for proper differentiation of body pattern, whereas trx acts predominantly in the zygote. Mutations in fsh and trx interact strongly, leading to high levels of homeotic transformations of a bithorax type. The fsh gene has been sequenced previously; in the past year a conserved domain has been discovered that is shared between fsh and several regulatory genes from Drosophila, yeast, and humans. This motif, named the bromodomain, may be involved in protein/protein interactions. The molecular analysis of the trx gene has been continued in two major directions. Antibodies prepared against fusion proteins of different parts of the very large trx open rading frame have detected several protein bands on Western blots. Immunocytochemical staining extends the earlier data obtained by in situ hybridization showing that trx is expressed in all cells of the embryo. The second direction involved further studies on the splicing pattern of the primary trx transcript. Evidence has been obtained for a complex pattern of multiply spliced RNAs derived from this locus.